1. Field of the Invention
The present invention relates to the random generation of a bit flow. The present invention more specifically relates to the generation of a high bit-rate flow (higher than 10 giga-bits/s) and more specifically applies to high-speed transmissions over communication connections or networks of any type.
2. Discussion of the Related Art
FIG. 1 very schematically illustrates in the form of blocks a first example of application of the present invention. It is a testing of a connection 1 of communication between a transmitter 2 (Tx) and a receiver 3 (Rx). The connection may be an electrical, optical, or microwave link. Communication standards provide standardized tests of simulation of the traffic on connections. Such tests are performed by means of a specific device 4 (TEST-RNG), connected instead of transmitter 2. This test device transmits a pseudo-random bit sequence PRBS over the transmission line. In the test application, an electrical, optical, radio, opto-electric, or electro-optic device (for example, the receiver or a clock recovery unit of the connection) may also directly be tested. The pseudo-random bit sequences are determined, for example, by a standard ITQ 0.151.
FIG. 2 illustrates in a very simplified view in the form of blocks a second example of application of the present invention. The case in point is to scramble or code a transmission, or to average the signal characteristics to mask the transmitted data or to balance the traffic over a connection.
FIG. 2 shows a transmitter 2 (Tx) connected to a connection 1. Transmitter 2 comprises a digital circuit 21 (μTx) for processing data D for their transmission after possible modulation (modulator 22) on a carrier OL originating from a local oscillator, and passing through a transmit amplifier 23 (LNA). A scrambler or coder 24 is provided at the output of circuit 21 before modulation by element 22. This scrambler (SCRAMB-RNG) aims at modifying, by means of a pseudo-random bit sequence, the characteristics of the transmitted data.
The invention also applies to an optical transmission. For example, a scrambler may be interposed upstream of the electro-optic conversion, the local oscillator being a light source, for example, a laser.
Pseudo-random generations are also used in error-correction code applications, transmissions of “code division multiple-access” type (CDMA), cryptography, etc.
FIG. 3 shows a conventional example of a generator of a pseudo-random bit sequence (PRBS) of the type used in the above-mentioned applications. Such a generator is based on the use of ring-connected shift registers. Several flip-flops 30 (B1, Bi, Bn) are series-associated, that is, the Q output of flip-flop B1 is connected to the D data input of the second flip-flop, and so on until the Q output of the penultimate flip-flop is connected to input D of the n-th flip-flop. The output of the last flip-flop Bn is looped back, via an XOR-type gate 31, onto the D input of the first flip-flop. The second input of gate 31 is connected to the output of an intermediary flip-flop Bi of the series association.
The number of flip-flops depends on the power desired for the pseudo-random bit sequence, that is, the number of bits on which the probability of obtaining a 0 or a 1 is respected. The longer the sequence, the greater number n of flip-flops, the better the random character of the generated PRBS sequence. Actually, the sequence length is equal to 2n−1. For example, by using 7 flip-flops, a 127-bit sequence is obtained.
The selection of the position of intermediary flip-flop Bi in the series association is linked to the obtaining of an irreducible polynomial of degree n and thus depends on the number of stages. The generated bit sequences are now called “m-Sequences” and respect a linear recurrence with a primitive polynomial characteristic. Such sequences are for example described in work “Finite Field For Computer Scientists And Engineers” of Robert J. Mc Eliece published by Kluwer Academic Publishers in 1995.
A disadvantage of current PRBS electric signal generators is linked to high bit-rate applications, that is, applications at several tens of gigabits/s. The forming of logic circuits and especially of fast flip-flops requires particularly expensive technologies. In practice, beyond from 10 to 20 gigabits/s, multiplexers having a number of inputs (and thus a complexity) linked to the desired acceleration factor must be used to mix phase-shifted signals according to an ETDM technique (Electrical Time Division Multiplexing). This solution further requires generating, in parallel, all the phase-shifted signals.
In an optical implementation, there now exists no equipment enabling achieving rates greater than 48 gigabits per second, unless multiplexers of OTDM type (Optical Time Division Multiplexing) in a number linked to the desired acceleration factor are used.